1. Field of the Invention
This invention relates to an evaporative fuel-purging control system for internal combustion engines, and more particularly to an evaporative fuel-purging control system for an internal combustion engine, which is adapted to control the flow rate of a gaseous mixture containing evaporative fuel purged into the intake system of the engine.
2. Prior Art
Conventionally, evaporative emission control systems have widely been used in internal combustion engines, which operate to prevent evaporative fuel (fuel vapor) from being emitted from a fuel tank into the atmosphere, by temporarily storing evaporative fuel from the fuel tank in a canister, and purging same into the intake system of the engine. Purging of evaporative fuel into the intake system causes instantaneous enriching of a total air-fuel mixture supplied to the engine. If the purged evaporative fuel amount is small, the air-fuel ratio of the mixture will then be promptly returned to a desired value, with almost no fluctuation.
However, if the purged evaporative fuel amount is large, the total air-fuel mixture supplied to the engine becomes very rich, so that the air-fuel ratio of the mixture may fluctuate. For example, a large amount of fuel vapor can be produced in the fuel tank immediately after refueling or fill-up. In order to prevent fluctuations in the air-fuel ratio due to purging of evaporative fuel (fuel vapor) on such an occasion, there has been proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 63-111277 a purging gas flow rate control system which reduces the purging amount of a mixture of evaporative fuel and air from the start of the engine immediately after refueling or fill-up until the speed of the vehicle in which the engine is installed reaches a predetermined value, and also reduces the purging amount of the mixture after the vehicle speed has reached the predetermined value and until the accumulated time period over which the vehicle speed exceeds the predetermined value reaches a predetermined value.
Further, an air-fuel ratio control system is also known e.g. from Japanese Provisional Patent Publication (Kokai) No. 62-131962, which forecasts an amount of possible variation of an air-fuel ratio correction coefficient caused by purging of a large amount of evaporative fuel, from an amount of variation of the air-fuel ratio correction coefficient actually caused by purging of a small amount of evaporative fuel, to thereby suppress fluctuation in the air-fuel ratio of the total mixture even when a large amount of evaporative fuel is purged.
However, the proposed conventional systems are liable to fail to perform accurate control of the air-fuel ratio since the actual flow rate of evaporative fuel is not detected by either of them in controlling the flow rate of a mixture purged.
Such inconveniences may be eliminated by providing a mass flowmeter in a purging passage and at the same time setting a desired flow rate of evaporative fuel based on operating conditions of the engine, whereby the opening of a purge control valve, which controls the purging, is controlled depending on an output value from the mass flowmeter and the desired flow rate of evaporative fuel to control the flow rate of the mixture purged.
According to this possible manner of eliminating the inconvenience described above, an accurate flow rate of evaporative fuel can be obtained since the flow rate of the mixture purged is directly measured by the flowmeter, which enables the air-fuel ratio control to be constantly effected in an accurate manner.
However, when the mass flowmeter becomes faulty or deteriorated in performance to output an abnormal value, the flow rate of the mixture purged is controlled based on such an abnormal value, which gives rise to the following problems:
If the output from the flowmeter indicates an abnormally small value, an excessively large amount of evaporative fuel is supplied to the engine in response thereto to cause the air-fuel ratio to be enriched to a large extent, which may result in stoppage of the engine or emission of noxious components, such as CO and HC, in large quantities. On the other hand, if the output from the flowmeter indicates an abnormally large value, an excessively small amount of evaporative fuel is supplied to the engine in response thereto to cause the air-fuel ratio to be leaned.
Further, in the above evaporative fuel-purging control, a vapor (evaporative fuel) flow rate-dependent correction coefficient for modifying the air-fuel ratio correction coefficient is calculated, and the opening of the fuel injection valves is controlled according to the fuel injection period calculated by the use of the air-fuel ratio correction coefficient thus modified. The vapor flow rate-dependent correction coefficient assumes a value inversely proportional to that of the flow rate of evaporative fuel. Therefore, if the output from the mass flowmeter assumes an excessively large value, the vapor flow rate-dependent correction coefficient becomes small to cause an insufficient amount of fuel injected, whereas if the output from the mass flowmeter assumes an excessively small value, the vapor flow rate-dependent correction coefficient becomes large to increase the amount of fuel injected, resulting in a largely enriched total air-fuel mixture. In both of the cases, the driveability or performance of the engine is degraded.